Common admin for WP + WP in subdirectory

I have just installed 2 WP in subdirectories, in addition to the one in root:

/public_html/
/public_html/1/
/public_html/2/

My question is it possible to have a common admin panel without multisite? Both GPT4 & Gemini keep telling me it is possible, but I no links actually state it is possible or how to do it.